This invention relates to signal transmission circuitry and methods. More particularly, this invention relates to pre-emphasis of data signals to improve signal transmission quality.
Signals transmitted at high frequencies and low voltages are particularly susceptible to signal losses over long traces. Traces are signal transmission paths through signal wiring, integrated circuit bus structures, PCBs (printed circuit boards), etc. Signal losses can be caused by, for example, attenuation, which is a decrease in the power of a signal, crosstalk, which is an adverse effect caused by signal transmission on an adjacent trace, and intersymbol interference, which is an adverse effect caused by residual noise from a previously transmitted signal. These losses can adversely affect the speed and accuracy at which transmitted data is received. For example, a logical 1 data signal may be incorrectly received as a logical 0 data signal and vice versa. An entire system can therefore be adversely affected by such signal transmission losses.
To compensate for such losses, signals may be “pre-emphasized.” Pre-emphasis is extra power (usually in the form of extra voltage, but extra current may be an equivalent) briefly applied to a transmitted signal immediately adjacent each signal transition (i.e., a signal changing from a logical 0 to logical 1 and vice versa). Pre-emphasis helps more quickly change the state of the medium transmitting the signal to receiver circuitry, and helps the receiver circuitry respond more rapidly to the change in state of the transmitted signal.
Pre-emphasis is becoming increasingly important as communication protocols and standards call for lower and lower signaling voltages (or currents) and increased signaling speeds. For example, very low signaling voltages are being specified for low voltage differential signaling (“LVDS”) and current mode logic (“CML”) communication protocols. A typical CML protocol may have a voltage swing of only 0.4 volts. At the same time, such a protocol may specify data transmission in the gigabit (i.e., billion bits) per second range. At such low voltages and high data rates, transmission line losses become a serious impediment to accurate and error-free reception of transmitted data.
Known pre-emphasis circuitries typically amplify a data signal at a constant amplitude level for the full duration of a baud period. A baud period can be generally thought of as the minimum amount of time between signal transitions. Such pre-emphasis, while improving signal transmission quality somewhat, does not adequately approximate the desired pulse shape of the transmitted signal. Thus, transmitted signals still lack the robustness desired for long traces and are accordingly still subject to transmission losses from, for example, crosstalk and residual noise. Moreover, because such known pre-emphasis is applied at a constant amplitude for the entire baud period, it results in high power consumption. Accordingly, known pre-emphasis circuitries can benefit from improvement.